Example run commands in Dockerhub readme don't work

从包含两个 docker run 示例的 Docker Hub README 部分开始，检查报告的 HAProxy 文件描述符错误。更新文档以讨论 --privileged 和 --ulimit，包括它们的权衡和替代方案，或者以其他方式使命令按描述正常工作。完成标准是：示例不再在没有解释的情况下失败，并且推荐 flag 的后果清晰明确。

Summary: The example run commands in the Dockerhub readme don't work, but they can be fixed by adding or discussing the --privileged or --ulimit flags.

The Dockerhub readme suggests the following docker run commands:

docker run -d --name my-running-haproxy --sysctl net.ipv4.ip_unprivileged_port_start=0 my-haproxy
# Or:
docker run -d --name my-running-haproxy \
  -v /path/to/etc/haproxy:/usr/local/etc/haproxy:ro --sysctl net.ipv4.ip_unprivileged_port_start=0 haproxy:2.3

When running either of these commands, HAProxy crashes with the following error message:

[ALERT] 040/170647 (8) : [haproxy.main()] Cannot raise FD limit to 8030, limit is 1024.

Running Docker in privileged mode lifts the 1024 limit on file descriptors. So that's one possible way to get past this error:

docker run -d --name my-running-haproxy --sysctl net.ipv4.ip_unprivileged_port_start=0 --privileged my-haproxy
# Or:
docker run -d --name my-running-haproxy \
  -v /path/to/etc/haproxy:/usr/local/etc/haproxy:ro --sysctl net.ipv4.ip_unprivileged_port_start=0 --privileged haproxy:2.3

Perhaps the readme should be updated to either:

1. discuss the --privileged and --ulimit flags, their pros, their cons, and alternatives; or
2. simply add --privileged to the example docker run commands.

Option 1 is probably better so as not to encourage use of --privileged without knowledge of the consequences.

I tested this against haproxy:latest and haproxy:alpine, which as of today map to HAProxy 2.3.5.